Apparatus for use in coating tubes internally with finely divided solid material



Nov. 19, 1963 A. P. BLACKBURN ETAL 3,111,429

APPARATUS FOR USE IN COATING TUBES INTERNAL-LY WITH FINELY DIVIDED SOLID MATERIAL 2 Sheets-Sheet 1 Filed April 21, 1961 INVEN TOKS HTTORNEYS Nov. 19, 1963 A. P. BLACKBURN ETAL 3,111,429

APPARATUS FOR USE IN COATING TUBES INTERNALLY WITH FINELY DIVIDED SOLID MATERIAL Filed April 21, 1961 2 Sheets-Sheet 2 3o Fig.2

INVNTORS HN 72mm Eszgg (JRA/ FITI'D RNEYS United States Patent APPARATUS FGR USE lN CfiA'l'll lt; TUBES 1N- TERNALLY Wl'lll FENELY DlWllEED Siilill) MATERIAL Alan Percival Blackburn, Shaw, and Edward Ernest Miles, Delph, England, assignors to The General Electric Company Limited, London, England Filed Apr. 21, 196i, Ser. No. 194,766) Claims priority, application Great Britain Apr. 29, 195i 7 Claims. (Cl. ll87) This invention relates to apparatus for use in coating tubes internally with a layer of finely divided solid material, and relates particularl though not exclusively, to apparatus for use in coating the tubular envelopes of low pressure mercury vapour fluorescent electric discharge lamps with a layer of powdered phosphor.

One form of apparatus which is in common use at the present time for the coating of tubular fluorescent electric discharge lamp envelopes comprises a closed vessel arranged to contain a quantity of powdered phosphor in suspension in a suitable liquid, and provided with a plurality of tubes extending downwards into the suspension from apertures in the closed top or" the vessel. The tubes communicate above the top of the vessel with rubber sleeves into which the lower ends or" vertically supported envelopes are arranged to be inserted, and when an envelope has been fitted into each of the sleeves the pressure within the vessel is increased to force the suspension up the tubes, the pressure then being slowly reduced so as to allow the level of the suspension to fall gradually and leave a coating of the suspension on the inner surface of the envelope; the suspending liquid is subsequently evaporated oil to leave a layer of powdered phosphor on the envelope wall.

With such apparatus the annular air space formed between the lower end of an envelope and its seating within the co-operating sleeve often causes eddies and bubbles in the suspension as it is forced up the tube, and this can lead, in many cases, to the production of poor quality phosphor coatings.

in addition, if an envelope does not seat properly in the respective sleeve, or fractures during the coating process, not only is there a loss of suspension from that tube, but the rest of the tubes on the apparatus are prevented from being properly coated.

The main object of the present invention is to provide an alternative form of apparatus which is particularly suitable for the coating of fluorescent discharge lamp envelopes and in which the above disadvantages are substantially avoided.

According to the invention apparatus for use in coattuhes internally with a layer of particles of a finely divided solid material comprises a conveyor incorporating a plurality of holders each arranged to support a tube with its axis vertical and to carry it from a loading to an unloading position, a trough located beneath said conveyor along at least part of the path of the tubes between the loading and unloading positions, means for connecting the top of each tube in turn to suction-applying means through a control valve whilst the lower end of the tube is held beneath the level of a coating suspension in the trough, for causing the suspension to be drawn up into the tube, a detecting means associated with each holder and arranged to efiect the closure of the respective control valve when the suspension reaches a predetermined height in the tube and means for admitting air at a controlled rate into the tube following the closure of the control valve for or using the level of the suspension to fall and leave a coating of the suspension on the tube Wall.

The detecting means conveniently comprises an electrical circuit incorporating a pair of spaced conducting "ice probes arranged to project downwards for a predetermined distance into a tube supported by the holder, and is arranged to effect the closure of the control valve when the probes are bridged by the suspension.

Where the suspension is sufiiciently conduc ing in such an arrangement the detecting circuit preferably incorporates a relay having its operating coil connected in series with the probes with respect to electric current supply means, the operation of which relay, when the probes are bridged by the suspension, effects the closure of the control valve.

in such an arrangement the control valve is preferably of the electromagnetically-operated kind having a control winding connected in series with the normally open contacts of an operating switch which is arranged to be closed automatically to open the valve when the respective holder reaches a predetermined position along its path of travel, the arrangement including also a hold-on circuit for maintaining the valve in the open condition when the operating switch is subsequently opened, and the detecting circuit relay having a pair of normally closed contacts connected in the hold-on circuit so that when the probes are bridged by the suspension the relay contacts are opened, thereby breaking the hold-on circuit and allowing the valve to close.

However, apparatus in accordance with the invention can also be used where the suspension is substantially non-conducting by employing a suitable form of detecting means. Thus where the detecting means is in the form of a circuit including a pair of spaced conducting probes arranged to project downwards into a tube supported by the respective holder, the circuit might for example, incorporate an alternating current electrical bridge with the probes connected in one arm of the bridge, and the arrangement being such that the change in the balance of the bridge, due to the change in the capacity between the probes when they are bridged by the suspension, gives rise to the closure of the control valve. Alternatively the detecting means mi ht incorporate a photoelectric device arranged to produce a control signal in response to a change in the incident radiation from a light source when the suspension reaches a predetermined height in the tube, for effecting the closure of the control valve.

The introduction of air into a tube after the closure of the respective control valve is preferably effected through a continuously-open orifice, the apparatus including, for each holder, an adjustable valve located between the orifice and the top of a tube mounted in the holder for varying the rate at which air is admitted into the tube and hence the rate of fall of suspension within the tube.

Similarly the pipe connecting a tube mounted within a said holder to the suction-applying means preferably includes an adjustable valve for varying the rate of rise of the suspension within the tube when the respective control valve is open.

The rate at which the suspension rises and falls within the tubes in use of the apparatus can then be adjusted to the most suitable values for the particular coating process for which the apparatus is employed.

In apparatus for use in coating the interior of fluorescent electric discharge lamp envelopes with a layer of powdered phosphor material a suitable rate for both therise and fall of the phosphor suspension is about 5 inches per second.

The suspension is conveniently arranged to be fed into the trough continuously at a substantially uniform rate from a reservoir of the suspension, an overflow pipe being provided on the trough for maintaining the level of the suspension within the trough substantially constant in use of the apparatus. Preferably the reservoir is mounted above the trough so that the suspension is fed to the trough under gravity; a pump can then be provided for feeding excess suspension back into the reservoir.

This enables the coating process to be carried out in a continuous manner which is an advantage over the pressure-feed type of apparatus previously referred to in which the coating process is interrupted periodically to enable the vessel containing the suspension to be opened and charged with a fresh supply of suspension.

Moreover since the suspension is not forced up all the tubes on the apparatus simultaneously as in the pressurefeed type of apparatus, only a relatively small quantity of suspension is required in the trough at any one time, and this is of particular advantage in the coating of fluorescent electric discharge lamps with powdered phosphor material in cases where water, possibly with the addition of a small quantity of a water-soluble binder, is used as the suspending liquid for the phosphor powder, especially where relatively small numbers of lamps are required to be coated with any particular phosphor material.

This is because the efficiency of a phosphor powder tends to deteriorate when kept for prolonged periods in water, so that after the coating of a batch of lamps has been completed the suspension remaining in the apparatus is often wasted, unless it is likely to be used again within a short period; by the use of apparatus in accordance with the invention having a trough of suitably small dimensions the wastage of phosphor can be considerably reduced.

Preferably the conveyor comprises a rotatable turret having the holders uniformly spaced around it so as to follow the same circular path as the turret rotates, the trough then being of annular form coaxial with the turret. The turret can either be rotated continuously at an appropriate speed in use of the apparatus, or it can be rotated intermittently so as to index the holders.

In use of apparatus in accordance with the invention for the coating of fluorescent electric discharge lamp envelopes the coated lamp envelopes are preferably transferred from the apparatus at the unloading position to a further conveyor arranged to support the tubes vertically, and incorporating means for blowing heated air through the envelopes as they are carried along by the conveyor to evaporate the suspending liquid, the envelopes subsequently being baked in accordance with known lamp making techniques for removing residual moisture and any binder which is present.

The transfer of the envelopes from the coating apparatus to the drying conveyor may, if desired, be effected automatically by any suitable form of transfer apparatus.

Similarly means can be provided for automatically loading the envelope on to the holders of the apparatus from another conveyor.

One form of apparatus in accordance with the invention, for use in coating the envelopes of low pressure mercury vapour fluorescent electric discharge lamps with an internal layer of powdered phosphor, will now be described by way of example with reference to FIGURES 1 and 2 of the accompanying drawings, in which FIGURE 1 represents in diagrammatic form a plan view of the apparatus, and

FIGURE 2 represents an axial section through the apparatus, also in diagrammatic form.

Referring to the drawings the apparatus incorporates a rotatable turret 1 carrying a plurality of holders 2 (only two of which are shown) spaced uniformly around it equidistant from the turret axis, the turret being arranged to be intermittently rotated, in use of the apparatus, by any sutiable drive mechanism 3 for causing the holders to index around a circular path in the direction indicated by the arrow in FIGURE 1. The envelopes 4 to be coated are arranged to be fed into the holders in turn at a loading position 5, the holders carrying the envelopes, with their axes vertical, to an unloading position 6 where they are removed from the apparatus.

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Each holder consists of a rubber sleeve connector 7 fitted on to the lower end of a vertical metal cylinder 8 slidably mounted in a guide 9 carried by the turret l. The upper ends of the envelopes 4 are arranged to be pushed into the open mouths of the connectors at the loading position, the connectors being a tight fit on the envelopes so that they support the envelopes and also provide a vacuum-tight seal. Spring clips it located beneath the holders 2 help to support the envelopes in the vertical position.

A metal tube 11 is supported coayially within each of the cylinders S by means of an annulus 12 of insulating material which hermetically seals the gap between the inner surface of the cylinder 8 and the outer surface of the tube. The cylinder-tube assembly is attached to a supporting member 13 carrying a roller 14 which rests on a cam rail 15 attached to a fixed support 3h.

The tube 11 of each holder 2 is connected at its upper end to a vacuum pump l6 through an individual electromagnetically-operated on otf valve 17 and a rotating distributor 13 which is common to all the holders.

Two conducting probes 19, carried by the cylinder 8 and metal tube Ill respectively project downwards through the connector '7 so that they extend a shoit distance into the upper end of an envelope fitted into the connector 7 as shown in FIGURE 2.

immediately beneath the circular path of the holders 2 there is located an annular trough 29 containing a sus pension of powdered phosphor material in an aqueous solution of a water soluble binder, the suspension being arranged to be fed into the trough at an approximately uniform rate in use of the apparatus from a reservoir 21. An overflow pipe 22 maintains a constant level of the suspension within the trough, excess suspension being fed back into reservoir by means of a pump 23.

The level or" the suspension in the trough is arranged to be about two inches below the bottom of an envelope 4 supported by a holder 2 at the loading position.

Then as the envelope 4 is carried round by its respective holder 2 the cam rail 15 lowers the supporting member 13 approximately three inches which causes the bottom of the envelope to enter the suspension in the trough 9 A micro-switch 24 carried by the turret 1 is then operated by a fixed cam 25 at the next position of indexing which opens the electromagnetically operated on-oif valve 17 and the distributor is employed to adjust the rate of rise of suspension within the tube to about 5 inches per second.

The on-off valve 17 is maintained in the open condition through several subsequent positions of indexing, even though the contacts of the switch 24 have opened, by means of a hold-on circuit provided by a relay R1 having its operating coil 25 shunting the valve 17 and a pair of normally-open contacts 27 shunting the switch 24. The suspension thereiore continues to rise gradually until it makes contact with the conducting probes l). The probes are connected in series with the operating coil 23 of a second relay R2 with respect to a source of electric current, and which relay has a pair of normally closed contacts 2: in series with the hold-on circuit of the relay R1. Then when the probes 19 are bridged by the suspension the relay R2 is operated, breaking the circuit through the relay R1 and through the eleotromagnetically operated on-ofi valve 7.7 and causing the valve to close.

An adjustable valve 31 connected in a branch tube between the valve 17 and the metal tube 11, and which branch tube is open to the atmosphere, permits air to enter the envelope at a controlled rate, and the level of the suspension within the envelope .then falls gradually at the rate of about 5 inches per second leaving a coating of the suspension on the envelope wall.

When the suspension no longer bridges the contacts, the relay R2 is de-energised and its contacts 2? close, the valve 17 remaining in the closed condition since the operating circuit thereof is broken by the switch 2 and the open contacts of the relay Rll.

After the level of the suspension has completely fallen the cam rail 15 raises the envelope once more and the excess suspension is allowed to drain from the envelope for a short while before the envelope reaches the unloading position 6. At the unloading position 6 the coated envelopes 4 are removed from their respective holders 2, the coating is dried by the passage of hot air through the envelopes whilst they are supported vertically, and the envelopes are subsequently baked to drive off any residual moisture and to remove the binder in accordance with known lamp manufacturing techniques.

A transfer device L of any suitable kind is conveniently provided for feeding the envelopes 4 automatically into the respective holders 2 from a conveyor C1 at the loading position 5, a further transfer device U being provided for removing the coated envelopes from the holders at the unloading position 6 and transferring them to a further conveyor C2 on which the drying of the phosphor coatings takes place. However, either one or both of the automatic transfer devices L or U may be dispensed with, and the loading or the unloading of the tubes or both then effected manually.

In the apparatus described each holder 8 may, if desired, be biased upwards to a rest position, in which the lower end of an envelope supported by the holder is clear of the suspension in the trough 20, by means of a spring, the cam rail 15 then being arranged to depress the holders against the spring bias for positively lowering the bottoms of the envelopes into the suspension at the appropriate position. After the coating of an envelope has been completed, and the level of the suspension within it has completely fallen, the cam rail 15 in such an arrangement is then arranged to permit the respective holder to return to its rest position, lifting the envelope clear of the suspension, under the action of the spring bias.

Also instead of the turret 1 being intermittently rotated the drive mechanism 3 might be arranged to rotate it at a constant annular velocity, the automatic loading and unloading means for inserting the envelopes 4 into and removing them from the holders, where such means are provided, being suitably designed for efiecting the loading and unloading of the envelopes whilst the holders are moving at constant speed.

We claim:

1. Apparatus for use in coating tubes internally with a layer of particles of a finely divided solid material comprising a conveyor incorporating a plurality of holders each arranged to support a tube with its axis vertical and to carry it from a loading to an unloading position, an openatopped trough for containing a quantity of a coating suspension, said trough being located beneath said conveyor along at least part of the path of the tubes between the loading and unloading positions, means for introducing the lower end of each of the tubes in turn into the coating suspension in the trough while the tube is carried between the loading and the unloading positions by the conveyor, means for connecting the top of each tube in turn to suction-applying means through a control valve whilst the lower end of the tube is held beneath the level of the coating suspension in the trough, for causing the suspension to be drawn up into the tube in use of the apparatus, a detecting means associated with each holder and arranged to effect the closure of the respective control valve when the suspension reaches a predetermined height in the tube and means for admitting air at a controlled rate into the tube following the closure of the control valve for causing the level of the suspension to fall and leave a coating of the suspension on the tube wall.

2. Apparatus according to claim 1 wherein the detecting means comprises an electrical circuit incorporating a pair of spaced conducting probes arranged to project downwards for a predetermined distance into a tube supported by the holder, and is arranged to eflect the closure of the control valve when the probes are bridged by the suspension.

3. Apparatus according to claim 2 for coating tubes internally with an electrically-conducting suspension wherein the detecting circuit incorporates a relay having its operating coil connected in series with the probes with respect to electric current supply means, the operation of which relay, when the probes are bridged by the 7 suspension, efiects the closure of the control valve.

4. Apparatus according to claim 3 wherein the control valve is of the electromagnetically-operated kind having a control winding connected in series with the normally open contacts of an operating switch which is arranged to be closed automatically to open the valve when the respective holder reaches a predetermined position along its path of travel, the arrangement including also a holdon circuit for maintaining the valve in the open condition when the operating switch is subsequently opened, and the detecting circuit relay having a pair of normally closed contacts connected in the hold-on circuit so that when the probes are bridged by the suspension the relay contacts are opened, thereby breaking the hold-on circuit and allowing the valve to close.

5. Apparatus according to claim 1 incorporating a reservoir for the suspension, means for feeding the suspension into the trough from the reservoir at a substantially constant rate, an Overflow pipe on the trough for maintaining the level of suspension within the trough substantially constant in use of the apparatus, and means for returning overflow suspension to the reservoir.

6. Apparatus according to claim 1 wherein the conveyor comprises a rotatable turret having the holders uniformly spaced around it so as to follow the same circular path as the turret rotates, the trough is of annular form coaxial with the turret, and the apparatus includes means for rotating the turret continuously at a substantially constant speed.

7. Apparatus according to claim 1 wherein the conveyor comprises a rotatable turret having the holders uniformly spaced around it so as to follow the same circular path as the turret rotates, the trough is of annular form coaxial with the turret, and the apparatus includes means for rotating the turret intermittently so as to indeX the holders around said circular path.

References Cited in the file of this patent UNITED STATES PATENTS 2,287,024 Casellini June 23, 1942 2,349,444 McGowan May 23, 1944 2,392,229 Cohen Ian. 1, 1946 2,449,783 Ladig et al. Sept. 21, 1948 2,728,686 Borushko Dec. 27, 1955 2,867,544 Hall Jan. 6, 1959 

1. APPARATUS FOR USE IN COATING TUBES INTERNALLY WITH A LAYER OF PARTICLES OF A FINELY DIVIDED SOLID MATERIAL COMPRISING A CONVEYOR INCORPORATING A PLURALITY OF HOLDERS EACH ARRANGED TO SUPPORT A TUBE WITH ITS AXIS VERTICAL AND TO CARRY IT FROM A LOADING TO AN UNLOADING JPOSITON, AN OPEN-TOPPED TROUGH FOR CONTAINING A QUANTITY OF A COATING SUSPENSION, SAID TROUGH BEING LOCATED BENEATH SAID CONVEYOR ALONG AT LEAST PART OF THE PATH OF THE TUBES BETWEEN THE LOADING AND UNLOADING KPOSITIONS, MEANS FOR INTRODUCING THE LOWER END OF EACH OF THE TUBES IN TURN INTO THE COATING SUSPENSION IN THE TROUGH WHILE THE TUBE IS CARRIED BETWEEN THE LOADING AND THE UNLOADING POSITIONS BY THE CONVEYOR, MEANS FOR LCONNECTING THE TOP OF EACH TUBE IN TURN TO SUCTION-APPLYING MEANS THROUGH A CONTROL VALVE 